I. Field of the Invention
The present invention relates to high temperature superconductor devices, and more specifically, to thick film superconductor devices having refined twin spacing and which maintain a large critical current.
II. Description of the Related Art
In recent years, extensive work has been conducted in the area of high temperature superconductors. Researchers have worked to develop materials which not only exhibit superconductivity at high temperature Tc, but which also are able to carry a large critical current density Jc. In particular, those skilled in the art have conducted numerous experiments with cuprate compounds in an attempt to develop superconductor devices which meet the foregoing criteria.
It is generally known that mechanical structures called “twins” form during certain crystal lattice transformations in cuprate compounds in order to reduce transformation strains. For example, in the article by A. Rosová et al., “Role of Microtwins in Twin Lamella Intersections and Interconnections in YBa2Cu3O7-y,” Physica, vol. C 214, p. 274 (1993), it is disclosed that mechanical twins form spontaneously in order to relieve strains during a ferroelastic phase transition when YBa2Cu3O7-d material is cooled in the presence of Oxygen. Rosová et al. disclose a technique for making a high temperature superconductor (Tc=92-93K) by preparing doped YBa2Cu3O7-d material, i.e., crystalline YBa2(Cu0.97Au0.03)3O7-d, in a gold crucible and annealing the material in oxygen to induce twinning.
Further discussions of the mechanical effects of twinning and the formation of secondary microtwins are presented in articles by Y. Zhu et al., “The Interface of Orthogonally Oriented Twins in YBa2Cu3O7-d,” Philosophical Magazine, vol. 67, A 1057 (1993), K. N. R. Taylor et al., “Intersections of Twins and of Optical Domains in Crystalline YBa2Cu3O7-d,” J. Crystal Growth, vol. 117, p. 221 (1992), and T. M. Shaw et al., “The Effect of Grain Size on Microstructure and Stress Relaxation in Polycrystalline YBa2Cu3O7-d,” J. Matter. Res., vol. 4, p. 248 (1989).
Although the phenomenon of twinning in YBa2Cu3O7-d superconductors is well documented, the mechanism by which twins impact superconducting behavior has been far less understood. It is known that when a current of density J flows in a mixed-state superconductor along a direction normal to an applied magnetic field B, the individual lines of magnetic flux are subjected to the Lorentz force, also called the Magnus force, F=J×B. Under most circumstances, as the applied magnetic field B increases, the superconducting current density J is degraded by the resulting Lorentz force. When there are few pinning centers to provide enough pinning, flux-lines will move with the Magnus force and there will be finite resistance and Jc will drop. However, in cuprate superconductors where twinning has occurred, lines of magnetic flux are “pinned” by the twin boundaries in the superconductor, and the magnetic flux is, at least to some extent, prevented from moving with the Lorentz force.
One article which examines the impact of twinning in YBa2Cu3O7-d superconductors is V. K. Vlasko-Vlasov et al., “Study of Influence of Individual Twin Boundaries on the Magnetic Flux Penetration in YBa2Cu3O7-d,” 72 Physical Review Letters 3246 (1994). That article discloses a YBa2Cu3O7-d high temperature superconductor formed by growing a YBa2Cu3O7-d crystal in a gold crucible. According to V. K. Vlasko-Vlasov et al., twin boundaries in the crystal act as pinning barriers which disrupt magnetic flux penetration of the superconductor, i.e., vortices of flux cannot easily pass through the boundaries and pile up on one side of the superconductor. Thus, with an applied magnetic field of 73 Oe, a critical current Jc=1.7×104 A/cm2 was measured by the authors in a twinned region of the superconductor as compared to an expected critical current of Jc=1.7×103 A/cM2 for an untwinned region.
Although V. K. Vlasko-Vlasov et al. explain that twin regions in a YBa2Cu3O7-d superconductor serve to disrupt magnetic flux penetration of the superconductor, this reference does not suggest any way of inducing or effectively utilizing such twin regions in a superconducting device.
Improvements in cuprate superconductor materials can be obtained by emphasizing and promoting the formation of secondary microtwin structures which facilitate strong flux pinning. This is described more fully in International Patent Publication WO 99/03159, published on Jan. 21, 1999, entitled “High Temperature Superconducting Device Having Secondary Microtwin Structures that Provide Strong Pinning,” which is incorporated by reference herein in its entirety.
One problem observed in superconductor materials relates to the properties of twins in thick film structures. As used herein, term “thick film” refers to coatings on a substrate which have a film thickness of 1 micron or more. In YBCO films less than 1 micron thick, such as 0.2 microns, twin spacings are generally small with respect to the film thickness and such films exhibit a desirably high critical current density. However, such films are not generally practical for many coated conductor applications, such as power transmission applications, where film thickness of 1 micron or larger are required. Previously, as film thickness was increased, the average twin boundary spacing has tended to increase as well. This results in a lower twin density and a drop off in the critical current density in superconducting films having a thickness of 1 micron or more.
Accordingly, there remains a need for improved methods and materials for thick film superconductor materials.